DS1 links, providing an access capacity of 1.5 Megabits per second (Mbps), dominate the existing access infrastructure. The next logical bandwidth step in the PDH/SDH hierarchy is DS3, which delivers an access capacity of roughly 45 Mbps. DS3 links, however, are unattractive to many enterprise customers since electrical DS3 solutions are capable of supporting access links of only about 900 feet in length, therefore mandating use of expensive optical fiber and optical network equipment for any access link spanning a significant distance. Accordingly, customers requiring more bandwidth than a DS1 link, found themselves reluctant to incur the expense associated with DS3 links. Therefore, it quickly became apparent that a better solution was needed.
Accordingly, several inverse multiplexing solutions have been proposed and implemented for aggregating two or more physical links into higher-bandwidth, virtual links. For example, link bonding solutions have been developed that attempt to solve these problems by aggregation of several DS1 links into one virtual or logical link at layer 2 or the link layer of the OSI reference model. Multiplexing refers to techniques for combining multiple data streams for transmission over a given high-speed circuit. In direct contrast, inverse multiplexing refers to techniques for using multiple circuits, such as T1 lines, behaving as a single logical link to transmit a given data stream. That is, inverse multiplexing divides a data stream or flow into multiple concurrent streams that are transmitted at the same time across separate physical links or channels. At the other end of these channels, the multiple data streams are then reconstructed back into the original data stream.
Inverse multiplexing schemes are known in the art. For example, the Inverse Multiplexing for ATM (IMA) Specification Version 1.1 (ATM Forum 1999), describes an inverse multiplexing protocol for transmitting ATM cell streams over multiple physical links and the retrieval of the original data stream at the far-end of the physical links. The ATM link aggregation solution was specifically designed to facilitate the aggregation of several DS1/E1 ATM links. The IMA solution was developed with certain main requirements in mind: 1) the ability to bond between 2 and 32 links; 2) the ability to bond both synchronous and asynchronous links; 3) the solution must be transparent to the ATM layer (and layers above the ATM layer); and 4) the solution must have carrier class features for stability, redundancy and fail-over.
For IMA, the third requirement ends up being the most taxing on the solution, because an ATM cell is a fixed size packet, and therefore does not allow for the simple addition of a sequence number to every cell to allow for re-sequencing of the data stream at the receiving end. The IMA solution instead introduced the concept of blocks, and establishes sequence integrity across the links by assigning sequence numbers to the blocks. A block is comprised of a fixed number of ATM cells, i.e. either 32, 64, 128 or 256. The default block size is 128 cells. IMA control cells define block boundaries and, among other things, include the block sequence number, and the offset of the control cell inside of the IMA block. The IMA scheme works by assigning a link priority at link initialization, and allocates the transmission stream in round-robin fashion equally among all links. The IMA scheme, uses the priority scheme to re-establish the correct cell stream sequence on the receiving end.
When the bonded links operate at different frequencies, however, the block alignments would slowly drift from each other. The IMA solution solves this issue by using cell-stuffing events. Specifically, the IMA frequency compensation scheme works by essentially introducing stuff events on the faster links, and thereby creating a nominal effective link bandwidth that is equal for all links (i.e., it effectively synchronizes all links to the slowest common denominator). The IMA solution identifies one link as being the nominal timing reference. This link has a cell-stuffing event introduced by default every 2048 cells. According to the IMA specification, the remaining links can have a cell-stuffing event introduced no more than every 5th super frame. These conditions limit the allowable difference in link speed among the bonded links. The amount of frequency offset between the links for which the IMA scheme can compensate depends on the nominal frequency of the links, and the IMA super frame size.
The IMA link bonding scheme can be used to bond other types of physical links, such as xDSL links. However, the minimal allowable link speed difference supported by IMA is a substantial obstacle against such an implementation. Even assuming this obstacle can be overcome, IMA still suffers from the inefficiencies introduced by the cell-stuffing events that bring the effective user bandwidth capacity of all links down to the lowest common denominator.
In light of the foregoing, a need in the art exists for link bonding schemes that efficiently utilize the available link capacity and more fluidly handle difference in link speed. Embodiments of the present invention substantially fulfill these needs.